Experimental study of hydrocarbon aviation fuel jets at high temperatures up to the critical point

Abstract

An experimental study was conducted to investigate the effects of fuel temperature up to the critical point on the injection and hydrodynamic characteristics of practical hydrocarbon aviation fuel jets. Pressurized heated hydrocarbon, simulating aviation fuel used as coolant in the active cooling system of a hypersonic flight vehicle, was injected through a set of plain orifice nozzles of exit diameters 0.7 and 1.0 mm. The fuel was heated to close to 700 K (427 °C) using an induction heater, and discharged to atmospheric pressure and temperature conditions. Visual observation of the fuel spray and the shock structure near the nozzle exit shows that morphological change is drastic in the temperature range between the boiling point and the critical temperature; this allows delineation of the liquid, flash boiling, and near-critical regimes. An in-depth investigation of the near-critical regime through shadowgraph images reveals that the structure of the hydrocarbon fuel jets near the critical temperature looks very similar to that of typical underexpanded jets. Furthermore, visual observation confirms that the distance hs from the nozzle exit to the shock structures is correlated with the injection pressure ΔP irrespective of the fuel injection temperature Tinj. Hydraulic characterization by measuring ΔP and the resultant discharge coefficient Cd with respect to Tinj under fixed mass flow rates shows that Cd is almost constant in the liquid regime, while it decreases with Tinj in the flash boiling regime, and that a discontinuity near the boiling point occurs where the liquid injection transitions to flash boiling. A simple linear model is proposed to estimate the P-T relation in the flash boiling regime by linearly extending the characteristics of the hydraulic performance.